Saturation monitor and process

ABSTRACT

A process for determining the degree of saturation of a solution comprising a solvent and a compound of limited solubility which involves the steps of (1) diluting a feed solution of the compound of limited solubility, (2) passing a sample of the diluted feed solution through a bed of a solid solution of the compound and a tracer in which the tracer is intrinsically incorporated into the crystal structure of the compound, so that the diluted feed solution being tested becomes saturated with the compound by dissolution of a portion of the solid solution in the bed, (3) measuring the concentration of the tracer in the sample after passage through the bed of solid solution, thus measuring the amount of the compound dissolved from the solid solution to saturate the diluted feed solution, and (4) determining the degree of saturation of the compound in the feed solution from the measured values

This is a division of application Ser. No. 301,647, filed Jan. 25, 1989,now U.S. Pat. No. 4,906,580, issued Mar. 6, 1990.

FIELD OF THE INVENTION

This application relates to a measuring process and system and is moreparticularly concerned with a process and system for monitoring thedegree of saturation of a solution comprising a solvent and a compoundof limited solubility in the solvent, especially the degree ofsaturation of an aqueous solution of gypsum.

BACKGROUND OF THE INVENTION

Methods and systems for measuring and monitoring solutions are known andinvolve a wide variety of techniques. For example, Hagedora U.S. Pat.No. 4,226,114 describes a method of analysis which is based uponmeasurement of change of enthalpy. Williams U.S. Pat. No. 4,680,271 usescontinuous differential colorimetric analysis. Both of these systems arebased on techniques which require certain physical or chemical changesto take place, are relatively complex and are apparently limited to usein connection with compounds which are highly soluble in the solvent inwhich they are measured. In the field of compounds of limited solubilityin a solvent, especially gypsum (calcium sulfate dihydrate) in water,particularly when several other chemical components are present, it hasheretofore been extremely time consuming to measure the degree ofsaturation or relative saturation of the compound of interest in thesolvent. Measuring relative saturation of a desired component of limitedsolubility is important in industry because, when such a solutionbecomes supersaturated with respect to a particular component, e.g. in aprocess in which the solution is evaporated, the component of limitedsolubility tends to come out of solution and to deposit on surfaces ofprocessing equipment (scaling), which leads to plugging and heat-tranferlosses and eventually to shutdowns, with concurrent loss in operatingtime. Two possibilities exist for a solution not in equilibrium withrespect to a given compound, either the solution is subsaturated . or itis supersaturated. The relative saturation is defined as being less thanone in the subsaturated case and greater than one when supersaturated.

In the field of gypsum-in-water solutions, encountered for example incertain gas desulfurization processes, wherein the solutions areevaporated, present techniques for determining the degree of saturationof gypsum in such complex aqueous solutions are, as mentioned, extremelytime consuming. For example, two approaches presently used to determinethe degree of saturation or relative saturation in solutions of thischaracter are (1) complete chemical analysis of the solution followed byuse of a computerized computational scheme, or (2) chemical analysis forcalcium and sulfate, equilibration with gypsum solids, and a reanalysisafter equilibration. Both of these techniques require significantanalytical capability and involve from four to eight hours to complete.

OBJECTS OF THE INVENTION

It is, accordingly, an object of this invention to provide an improvedmethod and system for monitoring the degree of saturation of a compoundof limited solubility in a solvent.

It is a further object of this invention to provide an improved methodof the character indicated which may be carried out in a relativelyshort time.

It is another object of this invention to provide an improved method formonitoring the degree of saturation of gypsum in aqueous solutions.

It is a still further object of this invention to provide a system formonitoring the degree of saturation of a compound of limited solubilityin a solvent.

BRIEF SUMMARY OF THE INVENTION

In accordance with the invention, there is provided a process whichcomprises (1) diluting a feed solution of the compound of limitedsolubility (compound of interest or the "target" compound) to be tested,(2) passing a sample of the diluted feed solution through a bed of asolid solution of the target compound and a tracer wherein the tracer isintrinsically incorporated into the crystal structure of the targetcompound, whereby the diluted solution being tested becomes saturatedwith the target compound by dissolution of a portion of the solidsolution (target compound plus tracer) in the bed, (3) measuring theconcentration of the tracer in the sample after passage through the bedof solid solution, whereby to measure the amount of target compounddissolved from the solid solution to saturate the diluted feed solution,and (4) determining the degree of saturation of the target compound inthe feed solution from the measured values.

BRIEF DESCRIPTION OF THE DRAWINGS

These and related objects and features of the invention will be apparentfrom the drawings wherein,

FIG. 1 is a diagrammatic view of a system suitable for carrying out theprocess of the invention; and

FIG. 2 is a diagram of the plot of illustrative measured points used indetermining relative saturation values in accordance with the invention.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

Referring to the drawings, and particularly to FIG. 1, there is shown asystem which includes a source 10 of the sample in which the relativesaturation of compound X in solvent Y is to be tested. The sample is aportion of the feed solution being tested which has been diluted with apredetermined amount of added solvent e.g water, in the case of asolution of gypsum in water. Sample source 10 is connected by a line 12to a valve 14 which valve 14 is connected by a line 16 to a pump 18. Asource 20 of a saturated solution of compound X in solvent Y is alsoconnected via line 22 to valve 14. Line 24 leads from pump 18 to asecond valve 26. Also included, in the system illustrated, is a column28 for containing a body 30 of the tracer-containing solid solution, anda measuring device or unit 32 which, in the embodiment illustrated, is aflow-through cell of a spectrophotometer. Column 28 is suitably providedat its top with an adjustable plunger 34 effective to reduce the headspace produced as the solid solution is gradually dissolved. Secondvalve 26 is connected to column 28 by a line 36 and column 28 isconnected to measuring device 32 by a line 38. A by-pass line 40connects valve 26 to line 38. The outlet from device 32 is indicated at42.

Thus, using the system of FIG. 1, a diluted sample of an original (feed)solution being tested is passed from the sample source 10 through thebed 30 of a solid solution of the target compound and a tracer containedin column 28 so that the sample becomes saturated with respect to thetarget compound accompanied by the tracer. Thereupon, the sample ispassed through a device for measuring the concentration of tracer in thesample, and the measured values are used in determining the degree ofsaturation of the target compound in the original (feed) solution beforedilution. The choice of tracer is determined by the ability to measureconveniently the tracer concentration and to form a solid solution withthe target compound. For example, if the tracer imparts a color to thesolution, as in the case of chromate, a flow-through cell in aspectrophotometer can be used to monitor continuously the tracerconcentration in the solution. The degree of saturation or relativesaturation of the original solution is determined on the basis of theconcentration of tracer measured and the level of dilution applied.

More particularly, the relative saturation monitoring process of thisinvention can be effectively used to determine the relative saturationof gypsum in solutions directly and rapidly without the need forsubsequent analysis of specific components. Thus, in the followingdiscussion, reference will particularly be made to gypsum and tochromate as a tracer for ease of description but, as previouslyindicated, the process of the invention is applicable to other compoundsof limited solubility in a solvent and to other tracers. The process ofthe invention, as mentioned, employs a tracer technique to measuredirectly the quantity of gypsum required to saturate a pre-dilutedsample, i.e. a pre-diluted sample of the original solution to be tested.The diluted sample is passed through a bed that is made up of a solidsolution of gypsum that has a tracer incorporated into the crystalmatrix. In this case, chromate is used as the tracer and deionized wateris typically used to effect dilution. The effluent from the bed flowsdirectly into a flow-through cell in a spectrophotometer where theconcentration of the dissolved chromate is measured colorimetrically.The quantity of gypsum dissolved is then calculated based on the knownquantity of chromate in the solid solution. The amount of chromate thatdissolves is directly proportional to the quantity of gypsum thatdissolves, since the chromate is bound into the gypsum crystal matrix asa true solid solution.

Conditions of flow rate and exposed surface area are selected such thatthe solution leaving the column will be saturated with respect togypsum. The amount of gypsum in the solid solution which dissolves tosaturate the diluted test solution will depend upon the solubility ofgypsum in that solution and the degree of subsaturation. In general, twoor more dilutions of the feed solution are needed to determine theoriginal degree of saturation. On the basis of the foregoing, the degreeof saturation can be considered as the reciprocal of the x-axisintercept of a line drawn through the data points which represent theamount of gypsum required to saturate the diluted samples as a functionof the dilution value.

In carrying out the process of the invention, a baseline is establishedby pumping a solution saturated with respect to the target compound e.g.gypsum, from source 20 directly to the measuring unit 32 e.g. aflow-through cell, by-passing the column 28 via line 40. Next, valve 14is adjusted so that the diluted sample is introduced into line 40 inplace of the saturated solution, i.e. still in the column by-pass mode,in order to establish any background absorbance.

In the case of chromate as a tracer, the absorbance is read at 338 nmwhich is the isobestic point for the chromate, dichromateinterconversion due to pH changes in samples. This minimizes the effectof pH on the absorbance, thus reducing the need to control the pH of thesamples.

The system is then switched by adjustment of valve 26 to the column modeand the absorbance is again determined, i.e. the sample is caused toflow through bed 30 in column 28. The procedure is repeated with atleast two different dilutions of the original solution to be tested.When samples are not being measured the saturated solution in source 20is pumped through the column 28 to purge out the previous sample and tostabilize the column.

In a particular embodiment, column 28 is 6 mm in diameter. The solidsolution 30 is packed into the column 28 to a height of about 20 mm. Theadjustable plunger 34 at the top of column 28 will reduce the head spaceproduced as the solid solution is gradually dissolved. The bed 30 ofsolid solution must be replaced periodically as the solid solution isconsumed. Suitably, the column 28 is repacked when approximatelyone-third of the original quantity of solid solution 30 has dissolved.

In this embodiment, the detector system is a Milton Roy SpectoMonitor3100 with a low-volume flow-through cell. The cell path length is 1 cmand has a volume of 0.014 mL. Suitably, the output of the monitor is fedto a strip-chart recorder for permanent record.

Ordinarily, the process of the invention is carried out at roomtemperature, (25° C.). It can, however, be carried out at othertemperatures, e.g. 40°-60° C., temperatures which are encountered in gasdisulfurization wherein gypsum solutions are formed, by enclosing thesample source and the column 28, for example, in atemperature-controlled oven

As will be apparent from the foregoing, an important feature of theinvention is the solid solution of the target compound and the tracer.Solid solutions are known and are prepared by techniques appropriate tothe compound and tracer involved. In the case of gypsum and a tracerwhich can be detected colorimetrically, e.g. chromium as chromate, thesolid solution is prepared, in accordance with the invention as follows:A solution containing K₂ CrO₄ and CaCl₂ in deionized water is heated toabout 50° C. and the pH is lowered to 6.5 to 7. A solution of K₂ SO₄ inhot deionized water is rapidly added to the original solution withvigorous stirring. The combined solution and resulting precipitate arestirred, e.g. for an additional 2 hours, with the temperature maintainedat about 50° C. The solids are then allowed to settle, the supernatantliquor is decanted, and the wet solids are repeatedly washed with asolution saturated with gypsum, and then dewatered. The solid solutionthus obtained contains 2-3% by weight of chromate and is suitably storedin a sealed container for eventual use in the saturation process of theinvention.

The invention will be further understood from the following example ofspecific application.

EXAMPLE

A solid solution of chromium (as chromate) in gypsum is prepared asfollows:

There are dissolved 360 g of K₂ CrO₄ in 1600 mL of deionized water in a4 L Erlenmeyer flask containing a stir bar for magnetic stirring.Stirring of the solution is begun. There are then dissolved 240 gCaCl₂.2H₂ O in 600 mL, and the resulting solution is added to the K₂CrO₄ solution. The second solution is allowed to cool to less than 50°C. before it is added to the K₂ CrO₄ solution. The pH of the combinedsolution is adjusted to between 6.5 and 7.0 with 1M HCl and it is heatedto 50° C. There are dissolved 180 g K₂ SO₄ in 1200 mL of deionized waterand the K₂ SO₄ solution is added to the CaCl₂ /K₂ CrO₄ solution, carebeing taken that the temperature of the K₂ SO₄ solution is not greaterthan 50° C. The combined solution is stirred for about 2 hours,maintaining the temperature between 40°-50° C. Heating and stirring arediscontinued and the solids are allowed to settle. The supernatantliquor is decanted and dionized water saturated with gypsum is added tothe solids, and thoroughly mixed. The solids are then allowed to settle,and the supernatant liquor is decanted. The addition of water, mixing,settling and decanting are repeated several times until the supernatantliquor shows a light yellow color.

The thus prepared solid solution, which contains gypsum and 2-3 weightpercent chromate, is then charged into a column 6 mm in diameter to aheight about 20 mm. A baseline is established by feeding a solutionsaturated with respect to gypsum directly to a flow-through cell (1 cmcell path length; volume 0.014 mL) of a Milton Roy Specto Monitor 3100.The output of the spectrophometer is calibrated by using the solutionsof known composition. Thus, a solution of deionized water saturated withgypsum is diluted to give solutions of 90%, 70%, and 50% saturationlevels. The amounts of gypsum to resaturate these solutions at 25° C.are 1.54, 4.61, and 7.68 millimoles of gypsum per liter. By measuringthese standards, a calibration of the spectrophometer output to theamount of gypsum dissolved is determined and the linearity of theresults can be evaluated. Using this calibration curve the amount ofgypsum that is dissolved to saturate the samples can be determined.

Operating at room temperature (25° C.), a diluted sample to be measuredis then prepared by diluting a portion of an aqueous gypsum solution tobe tested with deionized water to provide a 0.5 dilution ratio. Aportion of the diluted sample is then fed directly to the cell in orderto establish background absorbance. The absorbance is read at 338 mm.Next, the diluted sample is then fed through the column containing thesolid solution at the rate of 1-2 ml/min. The effluent from the columnis passed to the flow-through cell of the Milton Roy Specto Monitor andthe quantity of dissolved chromate in the sample measuredcolorimetrically. Thereupon the column is flushed with a saturatedaqueous gypsum solution to purge out the previous sample and tostabilize the column. A second and a third diluted sample to be measuredare then prepared by diluting other portions of the aqueous gypsumsolution being tested with deionized water to provide 0.7 and 0.9dilution ratios, respectively, these second and third samples are thenpassed through the column and into the flowthrough cell exactly asdescribed above in connection with the first diluted sample. In this waythe quantity of chromate (and gypsum) dissolved in each sample bypassing three different dilutions of the original gypsum solution beingtested can be determined. To indicate rapidly the degree of saturationof the undiluted solution, the amount of gypsum dissolved to saturatethe diluted samples as determined from the spectrometer measurements isgraphed against the dilution ratio. At least two different dilutionratios are needed, for each sample with three, as above, beingpreferable. The spectrophometer results obtained in this experiment aregraphed as a function of the dilution ratio, as seen in FIG. 2. A lineis drawn through the data points (squares) and extended to the x-axis.Thus, a curve (or straight line) is drawn through the data thatrepresent the best fit of the values and is extrapolated to the x-axis(zero gypsum dissolved from the solid solution). The dilution ratiovalue at this intercept represents the dilution of the original samplethat would have produced a solution at equilibrium with gypsum.Theoretical calculations of a wide range of solution compositions showthat the reciprocal of this dilution ratio is, for practical purposes,the relative saturation of the sample. The reciprocal of the dilutionratio value at the intersection of the extrapolated data-point line andthe x-axis gives the desired practical relative saturation value of theoriginal solution from which the samples were prepared. In the case ofthis experiment, the relative saturation value is found to be 1.06. Thedata points from this experiment are shown in FIG. 2 as squares, asmentioned.

It will be apparent that additional diluted samples can be measured andthat other dilutions can be employed. Moreover, when the tracer cannotbe measured colorimetrically, other appropriate measuring means can beemployed, following the principle of the invention which is based uponsaturation of a diluted sample by dissolution of a solid solution of atarget compound and a tracer and measuring the resultant amount oftracer (and added target compound) in the sample. The importance of theinvention is that it permits the treatment of solutions of compounds,such as gypsum, of limited solubility to determine rapidly their degreeof saturation in a practical, example illustrates the measurement of thedegree of solubility of gypsum in deionized water, it is a feature ofthe invention that it permits such determinations of a "target" compoundto be made even in the presence of other chemical constituents, such asare found in industrial practice, e.g. in gas desulfuration in the caseof gypsum. Thus, as seen in FIG. 2, data points for aqueous solutions ofgypsum also containing various concentrations of Mg Cl₂ as a contaminantare shown by circles and triangles. MgCl₂ is a contaminant known to havea significant effect upon gypsum solubility in water.

It will be obvious that various changes and modifications can be madewithout departing from the invention as defined in the appended claimsand it is intended, therefore, that all matter contained in theforegoing description and in the drawing shall be interpreted asillustrative only and not as limitative of the invention.

What is claimed:
 1. A method of making a solid solution of gypsum andchromium wherein about 2-3% chromate is substituted for sulfate in thegypsum crystal lattice, which comprises providing a first aqueoussolution of potassium chromate and calcium chloride at a pH of about 7,providing a second solution by dissolving potassium sulfate in heatedwater to effect complete dissolution, rapidly adding the second solutionto the first solution with stirring, allowing the solids to settle,decanting supernatant liquor, washing the solids with a saturatedaqueous solution of gypsum, and dewatering the washed solids.
 2. Amethod of making a solid solution as defined in claim 1, wherein the pHrange is 6.5-7.
 3. A method of making a solid solution as defined inclaim 1, wherein stirring is continued for about two hours after theaddition of the second solution to the first solution.
 4. A method ofmaking a solid solution as defined in claim 3, wherein the temperatureis maintained at about 50° C.
 5. A method of making a solid solution asdefined in claim 1, wherein the solids are repeatedly washed.
 6. Amethod of making a solid solution as defined in claim 1, whereinwashing, allowing the solids to settle and decanting are repeated aplurality of times until the supernatant liquor shows a light yellowcolor.